Since URLs are case insensitive, Special:Linksearch should be too.
It seems it currently is semi-case-insensitive, that is:
- [[Special:LinkSearch/ufbir.ifas.ufl.edu/Chap21.htm]] returns 1 result;
- [[Special:LinkSearch/ufbir.ifas.ufl.edu/chap21.htm]] returns nothing.
[[Special:LinkSearch/UfBiR.iFaS.uFl.EdU/Chap21.htm]] also returns the correct result. So, I suppose this only breaks after slashes (perhaps other special chars) in the url.